Diaphragms for electrolytic cells used to produce chlorine, and sodium hydroxide or potassium hydroxide from brine (hereinafter generally referred to as "chlor-alkali" cells) are conventionally asbestos fiber mat structures supported directly by the cathode of the chlor-alkali cell. Such asbestos diaphragms suffer the serious disadvantage of swelling under load, sometimes, for example, swelling up to 800 percent. Such swelling can result in filling the anode diaphragm gap, thereby increasing cell voltage and subjecting the diaphragm itself to attrition by gas released at the anode surface proximate to the swollen diaphragm. The result of severe swelling of the asbestos diaphragm and of attrition caused by gas released at the anode proximate the swollen diaphragm is to limit the lifetime of such diaphragms, used commercially, to approximately six months.
Many attempts and proposals for overcoming the problems of the conventional asbestos diaphragms involve polymer modification of the asbestos sheets. However, the majority of existing diaphragm-type cells are of complex geometric design; accordingly, the composite sheets are necessarily formed exterior to the cell and cannot be employed without significantly reducing the available diaphragm surface area. Moreover, such composite sheet diaphragms must be used in the filter press or "sandwich" type cell design to be useful.
One specific suggestion, for overcoming the problems of asbestos diaphragms, involves impregnating a preformed asbestos diaphragm with monomer or polymer and subsequently polymerizing the monomer in situ, or curing the polymer. However, such impregnation of asbestos fiber diaphragms results in the formation of continuous polymer coating on the surface of the asbestos fibers; the continuous polymer coating eliminates the water permeability properties of the asbestos fibers. Moreover, impregnation techniques which, by design or by accident, form a continuous skin on one surface of the asbestos, render the asbestos impermeable to the electrolyte.
Another proposal for overcoming the disadvantages of asbestos diaphragms involves a process which includes depositing a diaphragm from a slurry of asbestos fibers and polytetrafluoroethylene fibers on a foraminous cathode and heat treating the deposit to physcially bind it and to strengthen the diaphragm. However, the cost of these die-drawn polytetrafluoroethylene fibers of relatively large diameter are excessive and exceed the cost of fibrids as described in the instant invention. Likewise, the amount of such fibers required in the diaphragm for a given level of operation exceeds the amounts required when fibrids are utilized.